Hybrid-junction cut-off waveguide filter



Dec. 10, 1963 E. N. TORGOW 3,114,119

HYBRID-JUNCTION CUT-OFF WAVEGUIDE FILTER Filed June 16, 1959 2 Sheets-Sheet 1 INVENTOR EUGEN; N. TORGOW BY//nqn M,

ATTORNEYS Dec. 10, 1963 E. N. TORGOW 3,114,119

HYBRID-JUNCTION CUT-OFF WAVEGUIDE FILTER Filed June 16, 1959 2 Sheets-Sheet 2 "MA TCHED LOAD TU/VABLE CUT- OFF SECTION our Schematic diagram of a low pass filter F/XED V4/VE V4 IVE f Trough L ine Gen/er F in 1 ,155. 6'. mass SECTION OF COAX/AL LINE cu TOFF SECTION w/ 7'/-/ J7 g. 61' ADJUSTABLE RAD/AL VIM/E5 TU/VABLE THOUGH WAVE GU/DE JE F PS w/ TH r4 PERED CENTER Fl/V BYMMM ah ATTORNEYS United States Patent 3,114,119 HYBRID-JUNCTION CUT-(EFF WAVEGUIDE FETER Eugene N. Torgow, Jericho, N.Y., assignor to Polytechnic Institute of Brooklyn, Brooklyn, N.Y., a corporation of New York Filed June 16, B59, Ser. No. 820,749 8 Claims. (Cl. 333-41) This invention relates to wave filters for use in waveguides and in particular to band-pass and low-pass filters.

The invention involves the use of a waveguide hybrid junction and utilizes the cut-off principle of wavelength selection and rejection. Filters according to my invention are especially useful at frequencies above 10 kmc./

sec.

An object of the invention is to devise a wave filter of the band-pass or low-pass type of relatively simple construction by the use of cut-off sections of waveguide in conjunction with a biconjugate directional coupling device such as a magic-tee, a short-slot hybrid, or similar devices.

Among the possible advantages to be gained from the use of such filter structures are the simplicity of the design procedure and the ease of fabrication in those frequency bands where good hybrid junctions are available. Furthermore, the only important tolerance is on the widths of the cut-01f waveguide sections. As these widths are the largest critical parameters of the structure, the actual mechanical tolerances need not be excessively small. Filters of this type also are well suited for use in filter partitioning schemes as the matched loads terminating the cut-oil sections can be replaced by a shortslot hybrid or a similar device to recombine signals which would otherwise be absorbed by the loads. The recombined signals may then be fed through successive stages of drop-out filters. Furthermore, the input impedance to a hybrid cut-off filter is fairly well matched to the input line, even in the filter rejection band. Finally, the dependence of the cut-off frequencies on waveguide widths provide a technique for constructing simple variable pass-band filters.

A further object of the invention is to devise a waveguide section having a variable cut-oil characteristic whereby the band width of the filter may be varied.

Two forms of the invention are illustrated in the accompanying drawing in which FIG. 1 is a diagrammatic showing of one embodiment involving the use of a shortslot hybrid junction.

FIG. 2 shows a second embodiment using a magictee junction.

FIG. 3 illustrates a suitable arrangement for varying the cut-oil frequency of a Waveguide section forming part of the filter.

FIG. 3a is a sectional view of FIG. 3 along the line 3a3a; and

FIGS. 4, 5, and 6 show subject matter related to the instant invention.

Referring to FIG. 1, a conventional short-slot hybrid junction is formed by two waveguide sections it) and 11 coupled together through a short-siot formed in a common wall between the two sections. The four ports of the hybrid junction are designated 1, 2, 3 and 4 re sistive loads represented at R, R. Sections 10d and 11d have the same wi th a" to provide a cut-off frequency equal to or somewhat above f at the upper end of the band to be passed. Wave energy within the selected band f to f is taken off from port 4 and applied to a load represented by R When a signal is applied to the input port 1 of the filter through section 1017, only those frequencies above the cut-off of section 10b will enter the hybrid at port 1. In the ideal case, the power into the hybrid divides equally and appears at ports 2 and 3 with the signals at these ports degrees out of phase. If the signal frequency is below the cut-off of the guide sections Mid and 11d terminating these ports, the signal power undergoes complete reflection. Due to the 90 degrees differ ence in phase between the two signals reflected from ports 2 and 3, and the additional phase displacement each reflected signal experiences in traversing the coupler, the reflected signals arrive in phase at port 4 and in phase-opposition at port 1. Therefore, input signals having requencies between the cut-off frequency of the input guide section 11b and the cut-off frequency of the two line sections 10d and 11d connected to ports 2 and 3 will be transmitted to port 4 and will suffer no attenuation in the ideal case. Signals at frequencies above the upper cut-oil frequency f will arrive at ports 2 and 3, be transmitted through the two guide sections Nd and 11d and be absorbed by the resistive terminations to these two sections. Thus these signals will not appear at port 4 and the filiter has a band-pass characteristic.

FIG. 2 shows a similar filter employing a conventional magic-tee junction in place of the short-slot hybrid. The magic-tee is of conventional construction having an input arm 12, two balancing or conjugate arms 13 and i4, and an output arm 15. The ends of these arms present ports 1, 2, 3 and 4 corresponding to like-numbered ports in FIG. 1. Input energy is applied to port 1 through a cut-ofi waveguide section 12a connected to port 1 through a uniform taper section 1211, the input end of section 1217 being connected through a taper section to a supply section 12d of larger transverse dimensions. As in FIG. 1, the section 12a has the width a and the proper length to provide cut-oil. at a frequency equal to or somewhat below h, the lowest frequency in the band to be passed. Arm 13 is terminated in a cut-off section 13a connected to port 2 through a taper section 135, section 13a corresponding to cut-off section 10d in FIG. 1. And likewise, arm 14 is terminated in cut-oil section 14a which is connected to the arm through a taper section 14b, and cut-off section 14a corresponding to cut-off section lid in FIG. 1. Cut-off sections 13a and 14a are identical and provide cut-oil at or somewhat above the frequency f The basic principle of operation of FIGURE 2 difiers from the previous case in that there is no phase difference between the signals arriving at the conjugate arms 2 and 3. In order to cause signals reflected from these ports to emerge from port 4, it is necessary to reverse the phase of one of the reflected waves re-entering the This is accomplished by the introduction of a one quarter wavelength line section between one of the terminals or ports 2 or 3 and the corresponding cut-off terminating section. For example, section may be inserted between port 3 and taper section 14b and will have a length to produce a 90 degree phase shift at the mid-band frequency. This length of line has no effect on the performance of the filter in its rejection band, although the presence of this length of line will affect the performance of cascaded filters of this type.

Low-pass filter operation of FIGURES 1 and 2 can be obtained by omitting the cut-off waveguide section at the input to port 1.

The filter arrangement shown in FIGURES 1 and 2 may be made to cover bands of different widths and for different frequency limits by providing known arrangements for adjusting the cut-off frequencies of the input line sections 1011 and 12a, respectively, and for adjusting the cut-off frequency of the sections d, 11d, 13:; and 1411. One novel arrangement especially useful for this purpose is illustrated in FIGURES 3 and 3a. This arrangement will be described for use at 12a in the input a m 12 of FIGURE 2, but it will be understood that it may be used in the location of any of the other cut-elf sections of FIGURES 1 and 2.

As shown in FIGURE 3, the cut-off section 12a is of rectangular form having a width at and a height h. This section is provided at its ends with oppositely extendmg tapered sections 12b and 12c for connection in one arm of the hybrid junction, and a slot 16 is formed along the middle of the top wall of all three sections 12a, 12b and 12c. A metallic vane or plate 17 is mounted by structure not shown to move into and out of the slot 15 to produce a ridge effect in the three sections which effects a change in the cut-off frequency of the section 12a. As shown in the drawing, the lower edge of plate 17 has a straight parallel portion 1.7a extending throughout the length of the cut-off section 12a, and tapered portions 17b and 170 extending over the extent of line sections 1215 and 120, respectively. The tapered portions 17b and 170 prevent undesired reflections from the central portion 17a. Spring fingers 16a and 16b (FIG. arranged on opposite sides of slot 16 engage opposite faces of plate 17 to insure good contact between the plate and the top wall of the line sections.

I The cut-off frequency of the waveguide decreases as the depth to which the ridge is inserted into the wave guide is increased. Therefore a cut-off section is designed to have the cut-off at the highest required frequency when the ridge is completely removed from the guide.

Reference is made to a published description of my invention appearing in the Institute of Radio Engineers Transactions, volume MTT-7, January 1959, pages 163 to 167.

Other forms of the invention include the use of a strip transmission line hybrid ring with cutolf sections of trough waveguide as shown in FIG. 4-. Tunable filters can be obtained by using sections of trough waveguide whose cutoif frequency can be varied by adjusting the width of the center fin as shown in FIG. 5. Coaxial line hybrid rings can also be used with either variable trough waveguide cutoff sections or coaxial line cutolf sections of the type shown in FIG. 6. These forms of the invention are particularly useful at the lower microwave frequencies (500-5000 mc./sec.).

The term hybrid junction is used herein in a broad sense to apply to any 4-port junction, and is generic with respect to FIGURES 1 and 2.

I claim:

1. A band-pass waveguide filter comprising a hybrid junction having an input port, an output port, and two conjugate ports receiving energy equally from said input port, a cut-oif waveguide section connected to supply energy to said input port and having a cut-off frequency at the lower end of the band to be passed, identical cut-off waveguide sections connected to receive energy from said conjugate ports and having a cut-olf frequency at the upper end of the band to be passed, matched terminations for said identical cut-off sections for preventing reflection of wave energy above the cutoff frequency of said identical sections, said hybrid junction including means for efiecting transmission of wave energy reflected from said identical cut-off sections in like phases to said output port.

2, A filter according to claim 1 wherein said hybrid junction comprises a pair of parallel waveguide sections having a common wall and being coupled together through a slot formed in said common wall.

3. A filter according to claim 1 wherein said hybrid junction comprises a magic-tee junction.

4. A filter according to claim 1 wherein said cut-ofi sections embody means to adjust the value of the cut-off frequency.

5. A filter according to claim 1 wherein each cut-off section has a width smaller than the width of the arm of the hybrid junction to which it is connected, each cut-cit section being connected to the respective junction arm through a tapered section of the same height as the cut-oif section.

6. A filter according to claim 1 wherein each cut-off section has a Width smaller than the width of the arm of the hybrid junction to which it is connected, each cut-off section being connected to the respective junction arm through a tapered section of the same height as the cut-off section, the top walls of said cut-off and tapered sections having a slot formed along the center thereof, and a metallic plate mounted to move into said slot transversely of said sections, said plate having its en tering edge tapered over the portion which enters the slot in said tapered section.

7. In a wave filter, the combination of a main rectangular waveguide section of uniform transverse dimensions throughout its length, a tapered section coupled to the input end of said main section and being of the same height as said main section and of greater Width at its input end than said main section, the top walls of said sections having a slot formed along the center thereof, and a metallic plate mounted to move into said slot transversely of said sections, said plate having its entering edge tapered over the portion which enters the slot in said tapered section.

8. In a wave filter, the combination of a main rectangular waveguide section of uniform transverse dimensions throughout its length, an outwardly tapered section coupled to each end of said main section, each tapered section being of the same height as said main section and of greater width at its outer end than said main section, the top wall of said main section having a slot formedalong the center thereof and extending into the top wall of each tapered section, and a metallic plate mounted to move into said slot transversely of said sections, said plate having its entering edge tapered over the portions which enter the slot in said tapered sections.

References Cited in the file of this patent OTHER REFERENCES Sichak: Tunable Waveguide Filters, Proceedings of The I.R.E., vol. 39, No. 9, pages 1054059, September 1951.

Ragan: Microwave Transmission Circuits, vol. 9, (Pages 643-645 M.I.T. Radiation Laboratory Series. especially of interest), May 21, 1948, copyright McGraw- Hill Book Co., NY.

Rizzi: Microwave Filters Utilizing the Cutofi Effect, I.R.E. Transactions on Microwave Theory and Techniques, January 1956, vol. MTT-4 No. 1 (pages 36-40 relied on). 

1. A BAND-PASS WAVEGUIDE FILTER COMPRISING A HYBRID JUNCTION HAVING AN INPUT PORT, AN OUTPUT PORT, AND TWO CONJUGATE PORTS RECEIVING ENERGY EQUALLY FROM SAID INPUT PORT, A CUT-OFF WAVEGUIDE SECTION CONNECTED TO SUPPLY ENERGY TO SAID INPUT PORT AND HAVING A CUT-OFF FREQUENCY AT THE LOWER END OF THE BAND TO BE PASSED, IDENTICAL CUT-OFF WAVEGUIDE SECTIONS CONNECTED TO RECEIVE ENERGY FROM SAID CONJUGATE PORTS AND HAVING A CUT-OFF FREQUENCY AT THE UPPER END OF THE BAND TO BE PASSED, MATCHED TERMINATIONS FOR SAID IDENTICAL CUT-OFF SECTIONS FOR PREVENTING REFLECTION OF WAVE ENERGY ABOVE THE CUTOFF FREQUENCY OF SAID IDENTICAL SECTIONS, SAID HYBRID JUNCTION INCLUDING MEANS FOR EFFECTING TRANSMISSION OF WAVE ENERGY REFLECTED FROM SAID IDENTICAL CUT-OFF SECTIONS IN LIKE PHASES TO SAID OUTPUT PORT. 